what is engine "overrun" ?

This year as we all know at Silverstone there was the fuss over blown diffusers ? Horner and Whitmarsh got into a dispute,
and one of the things they talked about was engine overrun (particularly when Horner referred to the concessions made
to the mercedes powered cars ) . My question is what is engine overrun and how does it relate to blown diffusers (or more
specifically what was it about this concession the the mercedes powered teams about engine overrun that Horner
and Red Bull were not happy about) ?

Overrun would I guess relate to vehicle travelling with throttle lifted (or lift-off). You may have noticed on racing sportscars flames shooting out of exhaust pipes during lift-off or when dowshifting- and this is what Mercedes powered cars may have been utilizing to increase blown diffuser efficiency (one would assume the logical thing would be that engine would be tuned to burn as little fuel as possible in those conditions ie when the car braking, and I would venture a guess that Merc may have mapped engine differently- sacrificing some fuel efficiency in order to keep the flow and temperature of exhaust gasses on overrun similar to those in full throttle conditions, thereby increasing the downforce from diffuser under braking)... Well, that's my non-expert theory.

I think, you have the definition of "overrun" condition now.
As for the concessions made:

- there was argument as to "what defines closed throttle" - e.g. how many % of throttle/butterfly opening on the engine, relates to "Zero" % throttle input from the driver.
- F1 cars use a "drive by wire (aka fly by wire)" system for there throttle, which means there is no mechanical link between the throttle pedal, which the driver operates
and the actual throttle at the engine intake which meters the airflow through the engine.
- the common assumption would be, that if the drivers lifts his foot "off the throttle(pedal)", the Throttle(butterfly) at the engine goes to a fully closed position as well (or to a low % opening to maintain idle).
- this was not the case in F1, to keep airflow through the engine, teams would keep the throttle at the engine open (anywhere from 30-100%, this setting got more aggressive over time) while the driver was "off the throttle pedal".
- there where two applications for this concept, know as "cold blowing" and "hot blowing". (which refer to, if there was fuel burned during this condition or not)
- in the "cold blowing" case, the driver would lift off the throttle (pedal), and to reduce the power produced by the engine the teams would just cut the ignition &/or fuel injected into the engine, but keep the engine throttle open.
this would make the engine "pump cold air" out of the exhaust, like a compressor would do, to keep the airflow through the "exhaust blow diffuser" (EBD)

- now other teams, took up this idea, and extended it, by injecting fuel during this phase, the fuel would be either (self) ignite on hot parts of the exhaust valve and or exhaust system (which is what you see sometimes as "flames" out of the exhaust at Sports/GT/Touring Cars or in Rallying) or it gets ignited by the spark plug (via a special engine map(ping) ), very late in the working cycle of the engine (past TDC), so that it creates very little torque (mechanical power) - just heat and gas flow. (very low engine efficiency)

- Original the FIA declared a max. limit for engine throttle position, when the driver was "off the throttle(pedal), I´m not sure about the correct value, but if IIRC it was ~10-15%, just before the Silverstone GP.

- thereafter some teams argued, that they needed more opening because they claimed to use "cold blowing" mainly to cool their exhaust valves, and if they would be forced to run with lower settings they would suffer engine defects (remember that the engine design is "frozen" in F1)

- then others, came up with there own version of "reliability" problems, claiming they would need to "fire" during overrun ("hot blowing" ), because their engine was designed around this, and so the whole saga unfolded, as in how many % engine throttle opening was needed for safe operation of the engine, and wherever or not it was necessary to fire the engine during these conditions.

Maybe they started 'overcooling' them... You know, "we accidentally designed too efficient sidepod aerodynamics, causing too much flow throught radiators, and voila- the most simple and logical solution (not to mention cost effective) is the one that involves engine management".

I'm no engineer, but I've always thought overrun is simply when the car is pushing the engine, rather than the engine pushing the car.

I was under the impression it was called overrun b/c the term originated from the early days of racing when the engine would literally run over its rpm ceiling. For example, a Jag D-type is 83mm x 106mm and if it is capable of achieving 21m/s mean piston velocity, the max rev ceiling would be 6,000rpm (could be different than the recommended max rev limit for distance racing). Under power the engine couldn't rev above 6,000rpm, but an early upshift could cause the momentum of the car to push the engine revs beyond the 6000rpm max. It literally ran over the rpm limit it was capable of achieving at WOT. As I understand it, this definition slowly changed, and "overrun" was defined as anytime an downshift pushed the car beyond the red line on the dash (the recommended rev limit). It continued changing until it became what we know today--an engine being driven by the momentum of the vehicle.

This is just what I've heard, but I certainly wasn't alive to hang around the prototype paddocks in the 1950s.

I'm no engineer, but I've always thought overrun is simply when the car is pushing the engine, rather than the engine pushing the car.

I was under the impression it was called overrun b/c the term originated from the early days of racing when the engine would literally run over its rpm ceiling. .

A slight contradiction there, I think. That is surely over-reving the engine. I have always understood that over-run means, as you put it in your first sentence, that the car is moving, is in gear and the throttle is closed.

A slight contradiction there, I think. That is surely over-reving the engine. I have always understood that over-run means, as you put it in your first sentence, that the car is moving, is in gear and the throttle is closed.

Over-rev is on the throttle. Overrun pumps cold air, and overrun puts different stress on the engine and drivetrain.

I was just repeating a rumor I heard, which suggested the meaning of "overrun" has undergone numerous evolutions, and "overrun" was literally over run, during the bygone eras. Today we know overrun as off-throttle engine reciprocation with the car in gear, but even that definition is becoming outdated. With the introduction of electronic engine management, overrun describes the engine management program that controls the engine when the driver lifts. Therefore, the definition of overrun is different for every car, and different for a single car based upon the rev range and sophistication of the engine management.

I don't wish to spark a controversial anorak discussion, just relaying an interesting story I heard about overrun, and the concept about the evolution of the term.

I'm no engineer, but I've always thought overrun is simply when the car is pushing the engine, rather than the engine pushing the car.

I was under the impression it was called overrun b/c the term originated from the early days of racing when the engine would literally run over its rpm ceiling. For example, a Jag D-type is 83mm x 106mm and if it is capable of achieving 21m/s mean piston velocity, the max rev ceiling would be 6,000rpm (could be different than the recommended max rev limit for distance racing). Under power the engine couldn't rev above 6,000rpm, but an early upshift could cause the momentum of the car to push the engine revs beyond the 6000rpm max. It literally ran over the rpm limit it was capable of achieving at WOT. As I understand it, this definition slowly changed, and "overrun" was defined as anytime an upshift pushed the car beyond the red line on the dash (the recommended rev limit). It continued changing until it became what we know today--an engine being driven by the momentum of the vehicle.

This is just what I've heard, but I certainly wasn't alive to hang around the prototype paddocks in the 1950s.

I think you may want to replace "up-shift" with "down-shift" in your description of the "overrun" condition.Because as it reads now, it does not make sense - apologize.

In the good old days, overrun was used by the driver much more than today.
Changing gear with a stick lever perhaps with notchy levers connected to the selectors in the gearbox made it essential for the driver to master this difficult control system and balance its operation with throttle brake and clutch.
This 'driving' skill or art if you like rarely exists today with modern engineering systems.
Of course overrun still happens in modern performance cars but ABS and other electronics have reduced the need for fine control balance by the driver in a road car.
In F1 it is all done by the electronics automaticaly for gear changes in overrun.
It has to be, the gear shifts would be to fast for the driver to make manualy and at the same time balance the engine braking in overrun.

5. (Engineering / Mechanical Engineering) (intr) (of an engine) to run with a closed throttle at a speed dictated by that of the vehicle it drives, as on a decline

Not sure I would agree with that definition.
If the engine is taken in isolation, over run is the continued turning of the rotating parts when the motive force is removed.
What drives the engine and varies that force is an added factor as are the variables introduced by the rest of any powertrain.

Energy recover has nothing to do with overrun. Overrun is just screaming engine braking and that's what I meant.

Energy recovery can only happen with overrun.Without it there would be no energy gain.Engine braking is not overrun.Engine braking uses overrun energy to brake the vehicle.The RPM needed by the engine to give sufficient cold or hot diffuser blowing can be achieved using overrun energy.It can also be achieved using partial use of the four stroke cycle with combustion driving the engine.It all depends on the method used to control the whole powertrain.If you declutch the engine it is a simple matter to throttle the engine to give sufficient mass gas flow for the diffuser.With energy recovery using KERS, there is no longer the need for 'engine braking'.

Energy recovery can only happen with overrun. Meaningless, ICE can't recover energyWithout it there would be no energy gain.MeaninglessEngine braking is not overrun. WrongEngine braking uses overrun energy to brake the vehicle. Partially right, ICE is dissipating energy not generating it.The RPM needed by the engine to give sufficient cold or hot diffuser blowing can be achieved using overrun energy. Right, sort ofIt can also be achieved using partial use of the four stroke cycle with combustion driving the engine. Huh?It all depends on the method used to control the whole powertrain. Huh?If you declutch the engine it is a simple matter to throttle the engine to give sufficient mass gas flow for the diffuser. Wrong, this is not overrun if it is declutchedWith energy recovery using KERS, there is no longer the need for 'engine braking'. Wrong , ICE dont do KERS.

Energy recovery can only happen with overrun. MeaninglessTry recovering energy without slowing the vehicle.Without it there would be no energy gain.MeaninglessEngine braking is not overrun. WrongEngine braking is the result of overrunEngine braking uses overrun energy to brake the vehicle. Right.The RPM needed by the engine to give sufficient cold or hot diffuser blowing can be achieved using overrun energy. RightIt can also be achieved using partial use of the four stroke cycle with combustion driving the engine. Huh?With the clutch depressed the engine becomes a hot or cold air pump for use blowing the diffuser.It all depends on the method used to control the whole powertrain. Huh?Part KERS part engine braking part balancing the gear spacing.If you declutch the engine it is a simple matter to throttle the engine to give sufficient mass gas flow for the diffuser. Wrong, this is not overrun of course not in this case you would be using it to recover energy.With energy recovery using KERS, there is no longer the need for 'engine braking'. Wrong , ICE dont do KERS. huh

Energy recovery can only happen with overrun. MeaninglessTry recovering energy without slowing the vehicle.Without it there would be no energy gain.MeaninglessEngine braking is not overrun. WrongEngine braking is the result of overrunEngine braking uses overrun energy to brake the vehicle. Right.The RPM needed by the engine to give sufficient cold or hot diffuser blowing can be achieved using overrun energy. RightIt can also be achieved using partial use of the four stroke cycle with combustion driving the engine. Huh?With the clutch depressed the engine becomes a hot or cold air pump for use blowing the diffuser.It all depends on the method used to control the whole powertrain. Huh?Part KERS part engine braking part balancing the gear spacing.If you declutch the engine it is a simple matter to throttle the engine to give sufficient mass gas flow for the diffuser. Wrong, this is not overrun of course not in this case you would be using it to recover energy.With energy recovery using KERS, there is no longer the need for 'engine braking'. Wrong , ICE dont do KERS. huh

Since the disscusion addresses what happens to an ICE on overrun, what happens outside of it or while it's declutched is OT.

Since the disscusion addresses what happens to an ICE on overrun, what happens outside of it or while it's declutched is OT.

Gerrard, I think this is the point you're missing- overrun is not synonymous with deceleration. I would think that most KERS systems would be completely independent of ICE and therefore would work with car in neutral and engine happily idling...

As for OP's intent- I haven't seen him posting since, but I would guess that I have in my first post, in broad strokes, and further very informative explanation from TC3000 have addressed the topic to his liking.

Gerrard, I think this is the point you're missing- overrun is not synonymous with deceleration. I would think that most KERS systems would be completely independent of ICE and therefore would work with car in neutral and engine happily idling...

As for OP's intent- I haven't seen him posting since, but I would guess that I have in my first post, in broad strokes, and further very informative explanation from TC3000 have addressed the topic to his liking.

I understand exactly what you and TC3000 is saying.Applied to blown diffusers it makes sense in very broad terms.However the term overrun as applied to ground vehicles has always described the energy recovered from the vehicle inertia.Until recently this energy had almost always been absorbed through the powertrain and converted to pressure and heat in the engine to brake the vehicle.It is only recently and mainly in F1 that overrun energy has been used to power the engine and blow the rear diffuser.The term overrun had also been used to describe the continued rotation of a device when the energy moving it has been removed.Neither of these terms can be applied to the simple definition of using an ic engine solely to blow its exhaust gas over a diffuser.To achieve this a number of energy sources can be used.

Instead of another 25 posts of a fruitless pissing match over the definition of a word, a more interesting discussion would be of the OP's intent; what was being done in F1. There's a bunch of talk here of closed throttle... a closed throttle blows nothing. The idea was that the teams wanted to maintain "blown diffuser levels" of downforce during off-pedal moments. To do that you ideally want max RPM (more RPM = more volume/time). You want that steady maximum downforce whether you're braking hard, "coasting" under cornering, or accelerating hard. Jim Hall used an onboard snowmobile engine to keep downforce constant regardless of road speed. F1 teams were simply maximizing the blown diffuser element of downforce at all times by maintaining maximum "blow" regardless of the driver's foot. The FIA wanted the teams to match throttle opening to the instantaneous need for forward propulsion, but the teams wiggled out of that for the remainder of the season, apparently not even prepared to run an "honest" KERS.

The teams were making a mockary of the KERS idea. They were using fuel energy for fueled downforce, not forward propulsion. I think they were also using all that available powered time (not needed for propulsion) to charge the KERS... again making a mockery of the ER goal of KERS. Why upset the car with "generator braking" when you've got plenty of (more controllable) friction braking on the car, and you're going to be screaming the engine to keep exhaust flow maximised?

The debate/negotiations with the FIA were over what percentage of full throttle the teams could have while the driver was fully lifted. It doen't take much throttle opening to make a declutched engine run redline, but the teams wanted a lot... hence the suspicion they were charging the KERS with the IC engine, not the car's momentum.

Under this scenario the throttle pedal becomes an acceleration request rather than a throttle opening request. If the driver presses the pedal, the throttle is already be open blowing the diffuser and maybe charging the KERS. Seems to me the throttle ends up controlling the KERS generator load in addition to the transmission clutch and ratio selection. Oh yeah... and the throttle plate... almost forgot it. I'm assuming the brake pedal declutches the engine so RPM can be managed to be constant (high) to blow and charge. Of course all this has to be invisble to the driver, with perfect transitions off and back to propulsion.

Egineguy, you've confused me a bit here... Wouldn't it have been simpler to give the driver manually operated clutch, which he could've pressed during the most of the braking and avoid all that mockery by allowing him to rev the declutched engine with throttle? (or am I unaware of stability under braking issues that could've arisen from declutched engine?)

Egineguy, you've confused me a bit here... Wouldn't it have been simpler to give the driver manually operated clutch, which he could've pressed during the most of the braking and avoid all that mockery by allowing him to rev the declutched engine with throttle? (or am I unaware of stability under braking issues that could've arisen from declutched engine?)

Surely you jest. Drivers only have two feet! And three pedals would be just too confusing! Seriously, the driver IS pretty busy. No need to give him another task when it's easily automated to do a better job. Remember, he wouldn't just have to declutch, he has to maintain a constant RPM to maintain constant grip... if the RPM drops momentarily, grip drops and off ye go. If the KERS generator kicks in the RPMs would drop before he can react, etc. To do all this manually without automation probably wou;dn't work. I seem to recall word in the press that, at some point, McLaren was having trouble with loss of downforce when off throttle... perhaps before they fully automated/integrated the whole shabang?

Well, the thought of pedal never crossed my mind, honestly... A button on the wheel (maybe they could manage to squeezer in one), and enough throttle to keep rev limiter active at all times. Current system should have some degree of fluctuation in DF level as revs drop in each gear.

BTW, am I correct in assuming that even though it helps DF the whole idea of putting exhaust in high pressure area would adversely affect engine performance (by increasing the work spent to pump out exhaust gases)?

Instead of another 25 posts of a fruitless pissing match over the definition of a word, a more interesting discussion would be of the OP's intent; what was being done in F1. There's a bunch of talk here of closed throttle... a closed throttle blows nothing. The idea was that the teams wanted to maintain "blown diffuser levels" of downforce during off-pedal moments. To do that you ideally want max RPM (more RPM = more volume/time). You want that steady maximum downforce whether you're braking hard, "coasting" under cornering, or accelerating hard. Jim Hall used an onboard snowmobile engine to keep downforce constant regardless of road speed. F1 teams were simply maximizing the blown diffuser element of downforce at all times by maintaining maximum "blow" regardless of the driver's foot. The FIA wanted the teams to match throttle opening to the instantaneous need for forward propulsion, but the teams wiggled out of that for the remainder of the season, apparently not even prepared to run an "honest" KERS.

The teams were making a mockary of the KERS idea. They were using fuel energy for fueled downforce, not forward propulsion. I think they were also using all that available powered time (not needed for propulsion) to charge the KERS... again making a mockery of the ER goal of KERS. Why upset the car with "generator braking" when you've got plenty of (more controllable) friction braking on the car, and you're going to be screaming the engine to keep exhaust flow maximised?

The debate/negotiations with the FIA were over what percentage of full throttle the teams could have while the driver was fully lifted. It doen't take much throttle opening to make a declutched engine run redline, but the teams wanted a lot... hence the suspicion they were charging the KERS with the IC engine, not the car's momentum.

Under this scenario the throttle pedal becomes an acceleration request rather than a throttle opening request. If the driver presses the pedal, the throttle is already be open blowing the diffuser and maybe charging the KERS. Seems to me the throttle ends up controlling the KERS generator load in addition to the transmission clutch and ratio selection. Oh yeah... and the throttle plate... almost forgot it. I'm assuming the brake pedal declutches the engine so RPM can be managed to be constant (high) to blow and charge. Of course all this has to be invisble to the driver, with perfect transitions off and back to propulsion.

It is not 'a fruitless pissing match about the meaning of a word'.It is F1 biased aero engineering stealing yet another established definition in its continuing worship of downforce aero 'uber alles'.The rest of your post engineguy, I am in full agreement with. Good explanation of the wrong way to use energy recovery from an educated experienced person.The F1 teams (FOTA) completely altered the FIA regulations attempt to encourage new technology from the real world to be developed to its peak in F1.The result is that F1 is now nowhere close to that peak of world vehicle technology where it should be and continues to become more boring and of less interest as each new season unfolds.Many engineers and ideas people who have been in F1 for many decades are leaving the sport to find meaningful technical positions elsewhere.I blame them not.The new FIA F1 regulations for 2014 will either be a turning point or a death nell.Those who disagree can then join Ferrari and the soft drinks promoters in the stone age as new ideas for motorsport competition replace the historic.Patrick Head recently resigned from his position at F1 Williams.He had been complaining against electric traction using what has become his trade mark comment. 'Shock and Awe' to plead for the awesome noise to remain in F1. Strange that he has taken up a new position in the 'energy recovery' field and 'Shock and Awe' was a BBC series on educating people about electricity.Might tell you something about the future.